A variety of refrigerant compressors for use in vehicle air conditioning systems are currently available. A popular axial type compressor design includes multiple cylinders with double acting pistons. In this type of compressor, the cylinders are equally angularly spaced about and equally radially spaced from the axis of a central drive shaft. One set of such cylinders is provided at each of two opposing ends of the compressor. The double piston is mounted for reciprocal sliding motion in each set of opposed cylinders. Each piston is reciprocated by a drive plate, more commonly called a swash plate. During operation of the compressor, rotation of the drive shaft imparts a continuous wave-type reciprocating motion to the swash plate This driving of the swash plate in a nutating path around the drive shaft serves to impart a linear reciprocating motion to the pistons.
A thorough description of the operation of this type of compressor is disclosed in U.S. Pat. No. 4,360,321 to Copp, Jr. et al. (assigned to the assignee of the present invention) issued Nov. 23, 1982. In this compressor, the intake of refrigerant fluid into the cylinder and discharge therefrom is controlled by unidirectional reed-type valves located in valve plates at the ends of each cylinder. Annular intake and discharge chambers are provided in the compressor heads at each end of the compressor. A single port accommodates the transfer of fluid from the intake chamber to each cylinder bore, and a second port accommodates the transfer of fluid from each cylinder bore to the discharge chamber.
Improvements were previously made in this type of compressor by incorporating intake or suction ports into the ends of the pistons themselves. The ports are arranged in an annular array with equiangular spacing and at a constant radius from the longitudinal axis of the piston. Locating the intake suction ports in the ends of the pistons obviates the need for a separate intake chamber.
More particularly, during operation of the improved compressor, refrigerant fluid is communicated into the compressor and directed to the internal cavity or crankcase surrounding the swash plate, that is, on the back side of the pistons. As a piston begins its intake stroke, this refrigerant is suctioned through the ports in the piston into the cylinder bore defined between the piston and the discharge valve plate. As the piston then begins its discharge stroke, reed valves block the return flow of the refrigerant through the ports in the piston, thereby forcing it to discharge through the discharge port.
While this compressor design realizes several advantages over its predecessor, additional improvements are still possible For example, under certain operating conditions the improved compressor design may suffer from "slugging." Slugging occurs when lubricating liquid enters the cylinder bore or compression chamber (i.e. the region defined between the piston and the valve plate). As the piston begins its discharge stroke, it is forced to compress this liquid as well as the refrigerant gas in the chamber. Since the liquid is substantially incompressible, the discharge stroke of the piston is inhibited.
Additionally, in a compressor subject to slugging, the compressor components are subjected to higher loads and stress. The trapped liquid slugs cause simulated shock or impact loading, especially as the piston nears the end of its stroke. This action causes not only repeated excess force and torque loading on the components, but greatly increases the noise during operation. Accordingly, a need clearly exists for a design improvement to reduce the adverse effects of slugging.
The slugging problem primarily results from the relocation of the suction port assembly in the piston in the new design, referred to above. That is, the equiangular port placement around the head of the piston necessarily results in the deleterious condition in which liquid pooled in the lubricant reservoir at the bottom of the compressor crankcase is susceptible to being drawn directly into the cylinder bore. To explain further, tiny liquid lubricant droplets are interspersed throughout the refrigerant gas as a mist. This mixture is introduced into the crankcase to provide lubrication for the swash plate, bearings, and other internal components. Gravity causes the liquid particles to collect and accumulate at the bottom of the crankcase. Under certain operating conditions, the liquid lubricant level rises above the lowermost suction ports in the piston, or the lubricant splashes up during hard cornering, braking or the like. Consequently, as this piston reciprocates, this liquid is directly drawn from the crankcase reservoir into the cylinder bore.